This invention generally relates to storage casks, and is specifically concerned with a low cost, minimum weight cask for storing spent fuel assemblies on the facilities of a nuclear power generating station.
Casks for the transportation and storage of radioactive materials such as spent fuel assemblies are known in the prior art. Such casks generally comprise a cylindrical inner container integrally formed from cast iron, as well as an outer container which may be formed from steel. A plurality of radially extending fins is often provided around the perimeter of the outer container for dissipating the heat generated by the breakdown of the radioactive isotopes in the spent fuel. Additionally, a layer of neutron-absorbing material such as a high-hydrogen concrete or a polyurethane material is disposed between the inner and outer containers for absorbing any neutron radiation which may be emitted by the spent fuel assemblies. Finally, a removable basket assembly is typically provided within the interior of the inner container for both spacing and arranging the spent fuel assemblies disposed therein. In the prior art, such basket assemblies are formed from sheets of stainless steel which have been welded together to form an array of cells for receiving the spent fuel assemblies. To insure that no critical nuclear reactions will occur between adjacent fuel assemblies, these stainless steel sheets are often laminated with sheets of boron for poisoning any such reaction. Additionally, flux traps formed from two, spaced apart parallel plates are also provided between every interface of every two adjacent fuel assemblies to minimize the amount of thermal neutron flux radiated between the fuel assemblies.
In the past, such casks have been designed with the twin objectives of fulfilling both the storage and transportation criteria set forth by the Nuclear Regulatory Commission (NRC) in various federal regulations. In order to fulfill the storage criteria, the surface radiation of all such casks may be no greater than about 200 millirems per hour at any given point. Additionally, the cask must be capable of effectively rejecting the heat of decay generated by the spent fuel assemblies within it. If no effective heat rejection mechanism were provided, the temperature within the cask could become high enough to generate dangerous levels of pressure, particularly if water became present in the interior of the cask. In order to fulfill the transportation criteria, the NRC regulations maintain that the cask must be capable of withstanding the mechanical shock of a magnitude commensurate with that of a hypothetical vehicular accident that applies momentary forces to the cask of approximately 150 G's, simulated by dropping the cask from a distance of 9 meters upon a non-yielding surface. In this regard, it is not enough that the walls of the cask continue to contain the radioactive material after such a mechanical shock. They must further maintain water tightness at all points so that external water will not have an opportunity to leak into the interior of the cask and thermalize the neutrons being emitted by the spend fuel rods. Additionally, the basket structure within the cask must be capable of withstanding the approximate 150 G forces applied to its perimeter by the inner cask walls without any significant distortion of its individual, waste containing cells. If these cells did undergo such distortion, the effectiveness of the neutron traps installed between the cells could be jeopardized, which in turn might result in a criticality condition within the cask.
To simultaneously solve these two criteria, the walls of the inner vessels of prior art casks were both integrally-formed and cylindrically shaped so that they could withstand the high G forces. Additionally, the basket assemblies were made of large number of relatively thick stainless steel plates to withstand both the hypothetical impact load limit, and to provide the required neutron traps.
Recently, as more and more nuclear power plants are storing spent fuel assemblies on their own grounds, a need has arisen for a specialized, storage-only cask which is capable only of safely storing spent fuel assemblies on above-ground concrete pads. While the weight and structures of such casks should allow them to be easily locally portable on the grounds of the nuclear power plant, and while the surface radiation emanating from such casks should still be less than 200 millirem limit set by the NRC, their internal structure need not be capable of withstanding the high G limit associated with the hypothetical vehicular accident as such casks will not be transported outside of the facility. For such storage-only casks, a G-limit on the order of 20 to 40 G's is all that would be required; simulated by a controlled drop height of less than about one foot. Additionally, safety measures taken in the design of the basket assembly for transportation casks would not apply to storage-only casks.
While it would be possible to use a prior cask to merely store spent fuel assemblies on the grounds of a nuclear power facility the cylindrical shape of the thick iron inner vessel would render them less than optimally efficient with respect to the weight of the shielding materials used. Such inefficiency arises from the fact that the interior of the inner vessels of such casks is rectangular (or at least polygonal) to compliment the shape of the array of rectangular fuel assemblies disposed therein, while the outer walls are cylindrical. Since the maximum amount of permissible surface radiation for such cask is 200 millirems per hour maximum at each point on the cask, the radius of the inner vessel must be made large enough so that this maximum surface radiation level is not exceeded even at the points along the circumference of the cylindrical vessel where the walls of the vessel are the thinnest (which generally occurs at the corner of the rectangular array of fuel assemblies). This minimum shielding requirement in turn causes the walls of the cylindrical inner vessel to be necessarily thicker than they have to be at other points around the circumference of the vessel. In a full sized transportation and storage cask, the use of such cylindrically shaped inner and outer vessels can result in many tons of excessive and unused shielding material in the walls of this cask. Other weight inefficiencies result from the use of relatively heavy stainless steel in the basket assemblies, and the provision of the neutron flux traps between adjacent fuel assemblies. The net result of these two factors is that the basket assemblies used in the prior art are much heavier than they need to be for in-facility storage purposes. Such prior art basket assemblies are also incapable of accommodating a maximum number of fuel assemblies due to the space required for the flux traps. Hence a larger basket is needed when such flux traps are provided, which in turn increases the circumference (and hence the weight) of the surrounding shield walls. Still another shortcoming associated with the use of such prior art casks for in-facility storage purposes is the expense associated with their manufacture. The creation of a cylindrical inner vessel with integrally formed walls which has a rectangular or polygonal interior requires extensive amounts of expensive machining. Moreover, the welding together of the heavy, expensive stainless steel plates used in the basket assembly further adds a considerable amount of expense to the cask as a whole.